The present invention relates to an electrothermal energy storage system and more particularly to the use of carbon dioxide (CO2) in such energy storage system for direct storage and retrieval of energy.
Energy storage systems, such as electrical energy storage systems, use an excess of electric power to compress a gas, for example, air in a charging cycle, for storing the energy in a compressed fluid, and expand the compressed fluid in a discharging cycle, for driving a generator and retrieving the electric energy. However, such compressed air energy storage systems may require access to underground caverns for pressure storage of the compressed fluid which may not be economical, and thus have limited deployment.
Further, energy storage systems, such as thermal energy systems, may store thermal energy such as a solar energy in a molten nitrite salt, retrieve the energy to heat water or other working fluids such as carbon dioxide (CO2) and the like via a heat exchanger, and expand the vaporized fluid for driving the generator to generate the electric energy. However, such thermal energy storage systems are not presently suitable for electric energy storage due to low round-trip efficiency of conventional electric-to-thermal conversion technology, such as resistive heaters or heat pumps.
Thus, there is a need for an improved energy system for directly storing energy from multiple sources such as electricity and thermal energy, and for efficiently retrieving the stored energy and converting it to electricity.